The idea that electrons solids can be treated as particle-like objects has shaped much of modern condensed matter physics. Even in materials where electrons interact strongly, this assumption usually survives in the form of quasiparticles with well-defined energies and velocities. In a recent study led by Prof. Silke Bühler-Paschen at the Vienna University of Technology, this long-standing picture has been pushed beyond its limits. Her team has shown that a quantum material can display clear topological behavior even when electrons lose their particle-like character altogether, forcing a reassessment of how topological states of matter are defined.
Kirschbaum, D. M., Chen, L., Zocco, D. A., Hu, H., Mazza, F., Karlich, M., Lužnik, M., Nguyen, D. H., Larrea Jiménez, J., Strydom, A. M., Adroja, D., Yan, X., Prokofiev, A., Si, Q., & Paschen, S. (2026). Emergent topological semimetal from quantum criticality. Nature Physics. https://doi.org/10.1038/s41567-025-03135-w
In conventional metals, the flow of electricity is described as the motion of electrons that scatter from impurities and from one another. This picture also underlies most theories of topological materials, where robust electronic properties are traced back to the geometry of particle-like energy bands. These theories have been highly successful and have guided the discovery of materials with unusual and technologically promising properties. However, they rely on the assumption that electrons can still be described in terms of individual excitations, even if those excitations are strongly renormalized by interactions.
Prof. Silke Bühler-Paschen at the Vienna University of Technology, stated,
“We now know that it is worthwhile—perhaps even particularly worthwhile—to search for topological properties in quantum-critical materials. Because quantum-critical behavior occurs in many classes of materials and can be reliably identified, this connection may allow many new ’emergent’ topological materials to be discovered.”
There are extreme regimes in which this assumption breaks down. One of them is quantum criticality, which occurs near a phase transition at absolute zero temperature. In this regime, quantum fluctuations extend over all length and time scales, and the concept of long-lived quasiparticles ceases to apply. The material studied by Bühler-Paschen’s group, a compound of cerium, ruthenium, and tin, enters such a quantum-critical state at temperatures below one kelvin, where electrons can no longer be assigned a single energy or velocity.
Despite this, earlier theoretical work suggested that the same compound might host topological electronic states. This created a clear tension between theory and physical intuition, since topology had always been formulated using particle-based concepts. To resolve this contradiction, the researchers carried out precision transport measurements at ultralow temperatures, focusing on how electrical currents respond under different conditions.
The key experimental result was the observation of a spontaneous Hall effect. Normally, a Hall voltage appears only when a magnetic field deflects moving charge carriers. In this material, a transverse voltage emerged without any applied magnetic field, a hallmark of topological electronic structure. Strikingly, the effect was strongest precisely in the quantum-critical regime where particle-like behavior is expected to be absent. When external pressure or magnetic fields were used to suppress the quantum fluctuations, the Hall signal diminished and eventually disappeared.
This behavior indicates that topological properties can not only survive the breakdown of the quasiparticle picture, but may actually be tied to it. Theoretical work developed alongside the experiments shows that topological distinctions can arise from collective quantum fluctuations themselves, rather than from well-defined electronic bands. The resulting phase has been described as an emergent topological semimetal, emphasizing that its defining features appear only in the strongly correlated, non-particle-like state.
For materials science and engineering, the implications are significant. Topological effects are valued for their robustness against disorder, which makes them attractive for applications ranging from sensing to quantum information technologies. The new results suggest that quantum-critical materials, which are widespread in strongly correlated systems, could provide a new and largely unexplored platform for realizing such effects.
More broadly, the study shows that topology is a more general concept than previously assumed. While particle-based descriptions remain valid in many systems, they are not a fundamental requirement for topological behavior. As experimental techniques continue to probe matter under extreme quantum conditions, similar departures from established frameworks are likely to emerge, offering new directions for both fundamental physics and future device concepts.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
